Traditionally, the first four bars is like a call. Second four bars is a repetition of that call. Last four bars is an answer/response to the call, so lets separate it into those sections.

|| I | I | I | I || I | I | I | I || I | I | I | I ||

The answer should have some finality, so lets make a cadence.

|| I | I | I | I || I | I | I | I || V | V | I | I ||

Both calls are exactly the same. Thats boring. I usually say it more emphatically the second time, since they probably didnt hear it the first time. Well give a musical reason to draw in the listener.

|| I | I | I | I || IV | IV | I | I || V | V | I | I ||

Hmm, the answer seems a little dismissive, dont you think? Lets make it address the question a little more relevantly.

|| I | I | I | I || IV | IV | I | I || V | IV | I | I ||

Okay, now to bluesify it.

|| I7 | I7 | I7 | I7 || IV7 | IV7 | I7 | I7 || V7 | IV7 | I7 | I7 ||

Thats a basic blues form. There are a lot of different modifications that are frequently made, though.